1. Field of the Invention
The present invention relates to a transflective liquid crystal display device, and more particularly, to self adjustment of display brightness according to ambient lighting in a transflective liquid crystal display device.
2. Desciption of the Related Art
Liquid crystal display (LCD) devices are widely used as displays in devices, such as a portable televisions and notebook computers. Liquid crystal display devices are classified into two types. One is a transmissive type liquid crystal display device using a backlight as a light source, and another is the reflective type liquid crystal display device using an external light source, such as sunlight or an indoor lamp. It is difficult to decrease the weight, the volume, and the power consumption of the transmissive type LCD due to the power required by the backlight component. The reflective type LCD has the advantage of not requiring a backlight component, but it cannot operate without an external light source.
In order to overcome the drawbacks of these two types of LCDs, a transflective LCD device which can operate as both a reflective and transmissive type LCD is disclosed. The transflective LCD device has a reflective electrode in a pixel region, wherein the reflective electrode has a transmissive portion. Thus, the transflective LCD device consumes less than a conventional transmissive type LCD device because a backlight component is not used when sufficient ambient light is present. Further, in comparison with the reflective type LCD device, the transflective LCD device has the advantage of operating as a transmissive type LCD device using a backlight when no external light is available.
FIG. 1 is an exploded perspective view illustrating a typical transflective LCD device. The transflective LCD device includes upper and lower substrates 10 and 20 opposite to each other, and a liquid crystal layer 50 interposed therebetween. The upper substrate 10 is called a color filter substrate and the lower substrate 20 is called an array substrate. In the upper substrate 10, on a surface opposing the lower substrate 20, a black matrix 12 and a color filter layer 14 including a plurality of red (R), green (G) and blue (B) color filters are formed. That is, the black matrix 12 surrounds each color filter, in the shape of an array matrix. Further on the upper substrate 10, a common electrode 16 is formed to cover the color filter layer 14 and the black matrix 12.
In the lower substrate 20, on a surface opposing the upper substrate 20, a TFT “T” as a switching device is formed in shape of an array matrix corresponding to the color filter layer 14. In addition, a plurality of crossing gate and data lines 26 and 28 are positioned such that each TFT is located near each cross point of the gate and data lines 26 and 28. Further on the lower substrate 20, a plurality of pixel regions (P) are defined by the gate and data lines 26 and 28. Each pixel region P has a pixel electrode 22 comprising a transparent portion 22a and an opaque portion 22b. The transparent portion 22a is made of a transparent conductive material, such as ITO (indium tin oxide) or IZO (indium zinc oxide), and the opaque portion 22b is made of a metal having high reflectivity, such as Al (aluminum).
FIG. 2 is a sectional view of a conventional transflective LCD device, which helps to illustrate the operation of such devices. As shown in FIG. 2, the conventional transflective LCD device includes a lower substrate 200, an upper substrate 260 and an interposed liquid crystal layer 230. The upper substrate 260 has a common electrode 240 and a color filter 250 formed thereon. The lower substrate 200 has an insulating layer 210 and a pixel electrode 220 formed thereon, wherein the pixel electrode 220 has an opaque portion 222 and a transparent portion 224. The opaque portion 222 of the pixel electrode 220 can be an aluminum layer, and the transparent portion 224 of the pixel electrode 220 can be an ITO (indium tin oxide) layer. The opaque portion 222 reflects ambient light 270, while the transparent portion 224 transmits light 280 from a backlight device 290 disposed at the exterior side of the lower substrate 200. The liquid crystal layer 230 is interposed between the lower and upper substrates 200 and 260. Thus, the transflective LCD device is operable in both reflective and transmissive modes.
In order to obtain a stable display quality of the transflective LCD, it is desirable for the display brightness to also change when the ambient light of the environment changes. For example, when the ambient light becomes darker, the backlight has to become brighter to maintain the determined total display brightness. Contrarily, when the ambient light becomes brighter, the backlight intensity is decreased to maintain the determined total display brightness and reduce power consumption. Nevertheless, current transflective LCDs require manual adjustment to change the intensity of the backlight. This method of adjustment and is very inconvenient for users.
In U.S. Pat. No. 5,157,525, Eaton et al disclose an LCD device employing a photodetector to compensate for variation in the characteristics of the liquid crystal. The LCD uses a photodetector to detect the transmissivity of liquid crystal elements under the ON and OFF states. According to the signal from the photodetector, the voltage level of the pixel driving element can be adjusted to obtain an optimum contrast and brightness. Though effective, this method, nevertheless, does not disclose how to obtain optimum display brightness when the ambient light of the environment changes.